The invention relates to the manufacture of a measuring probe for a measuring head to detect the oxygen activity of metal melts, in particular steel melts. The measuring probe has a reference substance of known oxygen activity and which, upon submersion into the metal melt, is separated from the metal melt by means of a refractory solid electrolyte that conducts electrons reliably and conducts primaarily oxygen at higher temperatures and which permits the measurement of the electromotive force between the reference substance and a bath contact making contact with the metal melt.
Such measuring heads are known from DE-OS 29 00 069 and DE-PS 33 45 540. In conducting the measurement of the partial pressure of oxygen of a metal melt, such a head is mounted at the end of a lance to be submerged into the melt. The manner in which the partial pressure of oxygen is measured with such a measuring head by means of the electromotive force between the measuring probe and a bath electrode is described for example in the journal "Steel and Iron", Vol. 95, (1975), issue 22, page 84. The measuring head used there is explained in detail in the brochure "FERROTRON, Oxygen probe for metallic melts" of Ferrotron Elektronik GmbH, Ratingen (Federal Republic of Germany), issue 1, 1982. In this case the measuring probe, designed as a ZrO.sub.2 tube, can be provided with a coating that is inert with respect to the metal melt.
In order to improve the accuracy of measurement, in particular even in the case of aluminum-killed steels, attempts have been made to coat the measuring probe with platinum, but not with much success.
In is known from U.S. Pat. No. 4,135,012 how to etch the surface of ceramic products, manufactured on the basis of ZrO.sub.2, with acid in order to attain a permanent adhesion of a metal coating. The acids that are use in this process are concentrated sulfuric acid, ammonium bisulfate, alkaline metal bisulfate and mixtures thereof, and are used at temperatures of at least 200.degree. C.
According to U.S. Pat. No. 4,451,350, to improve the accuracy of measurement an attempt was made to provide the surface of the measuring probe with an aluminum layer containing a metal fluoride powder, whereby the wettability of the surface of the probe was to be increased. Yet this process, also, did not significantly improve the accuracy of measurement.
According to JP 62-157567 A (Token Sangyo), JP 60-177259 A (Sumitomo), JP 60-05763 A (Sumitomo), JP 56-100354 A (Hitachi), JP 56-100353 (Hitachi), JP 56-092450A and JP 84-044580 B (Hitachi), more or less good results were attained with surface coatings. However, the coating requires a relatively high cost of production, which increases the cost of the measuring probe. In addition, the surface coatings are susceptible to the attack of the metal melt so that their effectiveness is restricted.